feat: Solve missing-number problem

Author: hu6r1s

missing-number/hu6r1s.py

@@ -0,0 +1,6 @@
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        for i in range(len(nums)):
+            if i not in nums:
+                return i
+        return len(nums)